It is well known that expansion and contractions of muscle produce electrical signals that circulate upon the surface of a person's skin. Perhaps the most common are the expansions and contractions of the cardiac muscle, which produce electrical signals typically referred to as ECG signals. These ECG signals exhibit particular waveforms containing several distinct characteristics for each heartbeat. These characteristics, generally labeled P, Q, R, S and T, according to common medical usage, have allowed medical science to monitor a person's electrocardiogram and thus determine the heartrate.
Each heartbeat will produce an ECG signal that typically includes three "positive" peaks, labeled the P, R and T peaks. Usually, it is the R peak that is the largest of these three positive peaks. Since it is necessary that only one peak be detected for each heartbeat, a threshold detector can be employed in the simple case to distinguish between P and T waves, on the one hand, and R waves on the other. Accordingly, the R-wave peaks are available and often used to trigger the threshold detector to generate heartbeat count.
As noted at the outset, all muscle tissue will emit electrical signals when expanding or contracting, the heart being only one of the many muscle groups of a person's body. Thus, when a person is in motion, such as when exercising, the problem of monitoring the person's heartbeat by identifying particular characteristics of ECG waveforms becomes extremely difficult; the reason being that sensors placed on the person to detect the ECG signal also receive electrical signals, typically termed "artifact", produced by the other expanding and contracting muscles of the body. Often such artifact signals in the ECG waveform bear a marked resemblance to the R-wave peaks.
Under such circumstances, the practice of limiting the frequency response characteristics of the ECG waveform and/or rejecting artifact by amplitude discrimination or pulse width discrimination to identify each heartbeat has been found to be generally insufficient, even when these techniques are used in combination. Accordingly, in order to limit the artifact signals, produced when a person is in motion, it is desirable that the person remain relatively motionless while an ECG waveform is being obtained--particularly if the fidelity of the desired ECG waveform is to be as accurate as possible.
Yet, it is desirable that persons have the capability of somehow monitoring their heartbeat--primarily heartbeat rate-- while in motion. For example, one may wish to stress one's cardiovascular system within predetermined limits through exercise that produces cardiac activity (i.e., a heartbeat rate) greater than a predetermined minimum, yet less than a specified maximum. Of course, one could always cease exercising periodically and monitor heartbeat rate by monitoring his or her pulse. However, this technique is bothersome if exercise is to be sustained for any length of time (by jogging, for example) and can fail to warn one when cardiac activity reaches or exceeds a dangerous level during exercise.
Therefore, it is desirable that additional techniques be found to allow the identification of each heartbeat occurrence in an ECG signal produced while a person is, for example, exercising, so that such identification can, in turn, be used to provide accurate monitoring of a person's heartbeat. This is particularly true if the person wishes to exercise his or her cardiovascular system, yet maintain a heartbeat rate that is within predetermined limits.